A Post reporter spotted brown water from a rusty beam creeping down the wall of the front entrance of Citi Field’s main gate in Flushing, Queens, on 126th Street. The Mets are set to move there in April.

Rob Bedelis, a mechanical engineer who helped build the Milwaukee Brewers’ Miller Park, said Citi Field shouldn’t be rusting a little over two years after the start of construction.

“It’s a sign of the quality of workmanship,” he said. “If I were a fan, I wouldn’t be too thrilled. This might be cosmetic and not structural, but fans are paying a lot of money for baseball tickets.”